Process for etching by gas plasma

ABSTRACT

The invention relates to a process for etching a substrate with the aid of a gas plasma produced either by ultra-high frequency waves, or by ultra-high frequency and radio-frequency waves. 
     In this process, the gaseous medium used for the formation of the plasma comprises at least one non-carbon-containing fluorinating gas such as SF 6 , at least one rare gas such as Ar and at least one non-carbon-containing oxidizing gas such as oxygen, as well as optionally another gas such as nitrogen. 
     The invention is more particularly usable in the microelectronics field.

FIELD OF THE INVENTION

The present invention relates to a process for etching by plasma usablefor etching layers of a random nature (insulating, condutive,semiconductive) on random substrates (conductors, semiconductors,insulants) and in particular for producing large relief structures insilicon.

The invention more particularly applies to the microelectronics field,especially for producing magnetic recording heads and hybrid circuits,as well as in the optronics and sensor fields.

In known manner, for producing etchings by plasma, said plasma isproduced by reacting an appropriate gaseous medium with ultra-highfrequency and/or radio-frequency waves. The interaction of theultra-high frequency or radio-frequency waves with the gaseous mediummakes it possible to split up the gaseous medium into ionized species(ions, electrons) and into neutral species (atoms, molecules). Thesedifferent species constitute the plasma and initiate chemical reactionson the layer to be etched.

Thus, J. Electrochem. Soc., Vol. 132, December 1985 discloses an etchingprocess using a plasma produced by radio-frequency waves, in which thegaseous medium is constituted by a fluorinating gas formed by CF₄ orSF₆, to which is optionally added argon, oxygen or nitrogen. Thisdocument demonstrates that the addition of oxygen is prejudicial for theselectivity of the etching, because it increases the etching rate of themask and decreases the etching rate of the substrate, no matter what thefluorinating gas used. In addition, the gaseous medium recommended is amixed CF₄ /N₂ or SF₆ /N₂ system.

An apparatus making it possible to produce a plasma by ultra-highfrequency waves is e.g. described in French patent application FR-A-2534 040 and an apparatus making it possible to create a plasma byratio-frequency waves is e.g. described in French patent applicationFR-A-2 538 987.

U.S. Pat. No. 4,298,419 discloses a process for etching by plasmaproduced by ultra-high frequency waves, in which the gaseous medium isconstituted by a mixture of C₂ F₆ and H₂ an a.c. voltage in theradio-frequency field is applied to the substrate to be etched in orderto obtain a high etching rate when the substrate is insulating orcovered insulating or covered with an insultant. However, the etchingrates obtained with this process remain low, below 0.1 μm/min. In thisprocess, the plasma is produced by coupling between a waveguide and agas column without a preferred direction and focussed onto the substrateby creating a magnetic field. For high values of this magnetic fieldcyclotron resonance of the electrons is obtained, which increases thedissociation of the plasma. This requires the use of relatively lowpressures below 6.5 Pa (50 mTorr) and high pumping rates.

BACKGROUND OF THE INVENTION

EP-A-0 180 020 and Microelectronic Engineering, Vol. 3, No. 1/4, 1985,pp. 397-410 discloses plasma etching processes simultaneously usingultra-high frequencies and radio-frequencies with a gaseous mediumchosen from among halogens, rare gases, etc., in the case of EP-A-0 180020, or a mixture of CF₄ and oxygen in the case of the document.

These plasma etching processes simultaneously using ultra-highfrequencies and radio-frequencies generally make it possible to obtainan improved result compared with that obtained with plasmas producedsolely by radio-frequencies.

However, for producing deep structures in certain substrates, e.g. insilicon, in an industrial manner and with a good definition with respectto the geometry defined by the mask, it is necessary to further improvethe results, particularly the etching speed or rate in order to have ahigh industrial production rate adapted to the etched depth and thecontrol of the etched profile or anisotropy and the selectivity of theetching.

The characteristics of an etching made in a layer with the aid of aplasma are the speed or rate Vg, the anisotropy A and the selectivity Sof the etching. The anisotropy of an eching A is determined by the ratiobetween the maximum width of the etching produced beneath the materialconstituting the mask of the etching and the depth of the etching. Thesmaller the anisotropy A, the more anisotropic the etching. Theselectivity S of an etching corresponds to the ratio between the etchingrate of the layer to be etched and the etching rate of the mask of theetching. The higher the selectivity S, the better the selectivity of theetching.

SUMMARY OF THE INVENTION

The present invention specifically relates to an etching process makingit possible to obtain improved results, particularly with regards to theetching rate, the selectivity and the anisotropy, as a result of thechoice of an appropriate gaseous medium.

The inventive process for etching a substrate with the aid of a gaseousor gas plasma produced either by ultra-high frequency waves, or byradio-frequency and ultra-high frequency waves is characterized in thatthe gaseous medium used for forming the plasma comprises at least onenon-carbon-containing fluorinating gas, at least one rare gas, at leastone non-carbon-containing oxidizing gas and optionally at least oneother gas chosen from among nitrogen and chlorine containing gases, e.g.chlorine.

As a result of the choice of such a gaseous medium, not only is a highlydissociated plasma obtained, but also a plasma which is very rich inreactive species. Moreover, by regulating the composition of thismedium, it is possible to adapt its capacities to the requirements ofdeep etching.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In this medium, the non-carbon-containing fluorinating gas which can beused can e.g. be sulphur hexafluoride SF₆. It is also possible to usenitrogen trifluoride NF₃. Thus, the use of these fluorine-containinggases makes it possible to obtain a plasma which is highly dissociatedand very rich in reactive species. NF₃ can wholly or partly replace thesulphur hexafluoride, because it fulfils the same function as the latteras a fluorine-supplying reactive gas for etching and the nitrogenresulting from NF₃ decomposition is compatible with the etching process.

The rare gases which can be used can e.g. be helium, argon, neon andkrypton. The rare gas or mixture of rare gases used ensures thestability of the discharge and its extension to the substrate.

Thus, the dissociation of a rare gas leads to ionized and neutralspecies with a long life, which makes it possible to maintain for acertain time the discharges caused by the ultra-high frequency wavesoptionally associated with radio-frequency waves. In general, use ismade of argon or a mixture of argon and helium.

Thus, helium, which like argon, ensures the stability of the reactiveplasma with regards to the plasma propagation and the energy transfer tothe reactive species, reduces the temperature of the substrate comparedwith that obtained in the argon plasma alone, whilst ensuring a bettercalorie transfer into the medium. Thus, the thermal conductivity ofhelium at temperatures above 100° C. is much higher than that of argon.

The oxidizing gas which can be used is in particular oxygen, whichstimulates the formation of volatile products following thedecomposition of the fluorinating gas and following the reaction ofthese radicals with the surface of the substrate to be etched. It ispossible to use other non-carbon-containing oxidizing gases, e.g. N₂ O,which will decompose into nitrogen and oxygen, nitrogen being compatiblewith the etching process.

A gaseous mixture containing solely a non-carbon-containing fluorinatinggas such as SF₆, a rare gas such as argon and an oxidizing gas such asoxygen makes it possible to obtain a high reactivity of the discharge,even when the latter is produced solely by ultra-high frequencies. Thus,when using this process for etching silicon, it is possible to obtain asilicon etching rate of at least 10 μm/min, a very high selectivity withrespect to the SiO₂ mask greater than 100 and an etched depth uniformitybetter than 95%.

Moreover, the etching rate or speed is independent of the crystalorientation of the silicon Si<100> or <111>, as well as the type ofdoping and the dopant dose of the silicon and it is possible to passthrough a substrate with a thickness exceeding 500 μm. Moreover, acontrollable or isotropic etching profile with an alpha angle between50° and 70° is obtained, a surface state with a very limited roughnessfor significant etching depths and a silicon surface and of the SiO₂mask without any parasitic polymer deposition.

When using this gaseous mixture with a mixed ultra-high frequency andradio-frequency excitation plasma on a silicon substrate, the etchingspeed is substantially the same, namely approximately 8 μm/min, theselectivity relative to the SiO₂ mask is approximately 80 and the etcheddepth uniformity is 95%.

In the same way, the etching rate is independent of the crystalorientation of the silicon, the type of doping and the dopant dose. Itis possible to pass through a substrate thicker than 500 μm. Acontrollable alpha angle etching profile between 60° and 90° isobtained, i.e. a profile gain compared with the ultra-high frequencyplasma. The surface state has a very limited roughness below 0.1 μm andthere is no deposition of parasitic polymers on the surface of thesilicon and the mask.

These results are due to the choice of a gaseous medium simultaneouslyincorporating a fluorinating gas, an oxidizing gas and a rare gas. Onusing a plasma for etching obtained from only one of these gases, allthe advantages referred to hereinbefore, such as the etching rate,selectivity, uniformity and surface quality would not be obtained.

According to a variant of the invention, which can be used for etching asubstrate by a gaseous plasma produced either by ultra-high frequencies,or by ultra-high and radio-frequencies, the gaseous mixture formed fromthe non-carbon-containing fluorinating gas or gases, the rare gas orgases and the non-carbon-containing oxidizing gas or gases alsocomprises another gas for influencing the etching results.

This gas can be nitrogen, which makes it possible to improve the surfacestate of the etched layer leading to a 30% roughness reduction, whilstalso improving the etching homogeneity by approximately 10%. The effectof the nitrogen can be attributed to its function in the discharge as aproducer of species with a long life and low energy permitting a greatereffectiveness and a greater homogeneity of the discharge of the reactiveproducts.

The added gas can be a chlorine-containing gas such as chlorine, whichmakes it possible to improve the profile of the etched patterns,particularly in the case where the plasma is solely produced byultra-high frequencies, the alpha angle passing between 60° and 80°instead of 50° and 70°. The intervention of the chlorine on the alphaangle of the etching profile can be explained by the formation ofnon-volatile intermediate products of type SiCl_(x) with x≦3, byabsorption stages of the chlorine on the surface of the silicon and thenby the formation of a relatively stable product. This process inparticular develops on the walls of etched patterns receiving lessdirect ion bombardment from the plasma compared with the bottom of theetching. In the case where the etching is performed with a plasmaproduced by ultra-high frequencies and radio-frequencies, the ionbombardment phenomena are increased by the existence of a sheath levelwith the substrate, which aids the bombardment perpendicular to thebottom of the etching. As a result of this bias, etched profiles closeto 90° are obtained.

When, according to the invention, a substrate is etched by means of agas plasma produced by ultra-high frequencies and radio-frequencies, onthe substrate confinement takes place by a cell made from a materialtransparent to the ultra-high frequency waves, of a gas plasma producedby transverse monomode coupling between a waveguide traversed byultra-high frequency waves and the cell in which at least part of theetching gaseous medium circulates perpendicular to the ultra-highfrequency waves for being directed onto the substrate and simultaneousapplication takes place to the substrate of a d.c. or a.c. voltage witha frequency in the radio-frequency range.

This plasma etching process simultaneously using ultra-high frequenciesand radio-frequencies makes it possible to obtain a great effectivenessof the dissociation of the gases in the vicinity of the substrate to beetched, because the dissociation efficiency is directly linked with theelectron density of the plasma, which is approximately 100 times higherin this case than in a discharge induced in the same gas by aradio-frequency device.

However, the process according to the invention also makes it possibleto obtain good results with a gas plasma produced solely by ultra-highfrequencies. In this case, confinement takes place on the substrate by acell of a material transparent to the ultra-high frequency waves of agas plasma produced by monotransverse coupling between a waveguidetraversed by ultra-high frequency waves and the cell in which at leastpart of the etching gaseous medium flows perpendicular to the ultra-highfrequency waves to be directed onto the substrate.

In both cases, etching takes place under a pressure of 0.1 to 300 Pa andpreferably 1 to 100 Pa. In both cases, when the gaseous medium is formedby three gases such as SF₆, Ar and O₂, the respective flow rates ofthese three gases are in the following ranges:

4-500 cm³ standard/min for SF₆,

4-500 cm³ standard/min for Ar,

2-500 cm³ standard/min for O₂,

i.e. pressures of:

0.27 to 67.5 Pa,

0.27 to 135 Pa,

0.13 to 67.5 Pa,

which corresponds to a total pressure of 0.67 to 270 Pa.

When the gaseous medium is formed by four gases such as SF₆, Ar, O₂ andN₂, the respective flow rates of these gases are in the followingranges:

4-500 cm³ standard/min for SF₆,

4-500 cm³ standard/min for Ar,

1-300 cm³ standard/min for O₂,

1-300 cm³ standard/min for N₂,

i.e. pressures of:

0.27 to 67.5 Pa for SF₆,

0.27 to 135 Pa for Ar,

0.13 to 67.5 Pa for O₂, and

0.13 to 67.5 Pa for N₂,

which corresponds to total pressures of 0.53 to 270 Pa.

It is pointed out that within the scope of the present invention asubstrate is understood to be both the material alone and asuperimposition of layers of different materials, etching in this casebeing performed on the final layer, which may or may not be protected bya mask. Preferably, the cell made from the material transparent to theultra-high frequency waves issues above the substrate by means of aflared or widened part.

Thus, when using a gas plasma produced by ultra-high frequencies andradio-frequencies, it is possible to obtain high etching rates,particularly when the substrate or the upper surface thereof is aconductive or semiconducting material, e.g. polycrystalline silicon.

This is in particular due to the way in which the plasma is created andconfined. Thus, through using a propagative plasma directed onto thesubstrate by a cell made from a material transparent to theelectromagnetic rays issuing onto the substrate by a flared part, makesit possible to confine the plasma on the substrate and operate atpressures higher than those used in U.S. Pat. No. 4,298,419. Byoperating at higher pressures, it is possible to obtain higher gas flowrates, e.g. 100 cm³ standard/min and thus create more reactive species,which makes it possible to increase the etching rate by a factor of 10.Thus, with the process according to the invention operating under apressure of 13.5 Pa, it is possible to reach a silicon etching rate ofat least 15 μm/min, whereas in the process of U.S. Pat. No. 4,298,419 itis only possible to obtain silicon etching rates of approximately 0.015μm/min.

In the process according to the invention, as a result of thepropagation mode of the microwave plasma and the coupling mode betweenthe plasma and the ultra-high frequency wave, the application of an a.c.or d.c. voltage to the substrate makes it possible to improve theprofile of the etching.

The application of an a.c. voltage to the substrate creates in thevicinity of the latter a sheath opposing the arrival of reactive speciesof the microwave plasma and consequently reduces the selectivity andetching rate of a semiconductor material such as silicon, whilstincreasing the anisotropy of the etching profile.

In the process of the aforementioned U.S. Patent, when the substrate isof an insulating material or has a surface insulating layer, e.g. ofSiO₂, a higher etching rate is obtained than in the case of silicon,whereas a reverse phenomenon is obtained with the process according tothe invention on operating at a pressure of approximately 10 Pa, becausein this case the etching rate of the silicon dioxide is lower than thatof silicon.

In the process according to the invention, the ultra-high frequencywaves are coupled with the plasma within the cell made from a materialtransparent to ultra-high frequency waves. Thus, an equilibrium iscreated between the plasma, which has its own energy characteristics,and the ultra-high frequency waves. The latter are transmitted directlyto the plasma via the waveguide and the cell. Moreover, there is no needto use a magnetic field for strongly ionizing the gaseous medium.

In the process of the aforementioned U.S. Patent, the ultra-highfrequency waves produce the medium excited at the end of theirpropagation lines located on the discharge chamber and the presence ofthe magnetic field makes it possible to confine the dissociated specieson the substrate in order to avoid their recombination on the walls ofthe discharge chamber.

Thus, with the process according to the invention, it is possible toobtain a better silicon etching selectivity compared with a SiO₂ maskand, for obtaining a given profile, it is possible to have a betteretching rate (at least 10 μm/min) and a better selectivity, generallyhigher than 100 when operating under 10 Pa.

The process according to the invention e.g. makes it possible to etchinsulating materials, such as organic materials (polyimides, resins,etc.), semiconductor materials such as polycrystalline, monocrystallineor amorphous silicon, III-V compounds and conductive materials such astungsten, molybdenum, tantalum, niobium, titanium and their silicides.

The process according to the invention makes it possible to obtain acompromise between the selectivity S, the rate Vg and the anisotropy Aof the etching obtained with plasmas produced either by ultra-highfrequency waves or by radio-frequency waves. The process according tothe invention makes it possible to improve the rate Vg and selectivity Sof a process for etching by a plasma produced by radio-frequency waves,whilst retaining a good anisotropy A of the etching. Therefore theinventive process is of particular interest for producing deep etchings,such as e.g. for producing magnetic recording heads.

For realizing the process according to the invention with a plasmaproduced by ultra-high frequency and radio-frequency waves, theultra-high frequency and radio frequency waves are establishedindependently of one another. The radio-frequency waves initiate thecreation of the plasma and consequently enable to ultra-high frequencywaves to be coupled with the plasma in order to also contribute to thecreation of the latter.

So that there is no parasitic interaction between the ultra-highfrequency waves and the radio-frequency waves, the interaction of theultra-high frequency waves with the gaseous medium takes place at adistance from the interaction of the radio-frequency waves with thegaseous medium. This distance is e.g. approximately 7 cm. The distancebetween the two interaction types makes it possible not to disturb thepropagation of the ultra-high frequency and radio-frequency waves and toensure a good superimposition thereof in the gaseous medium.

The frequency of the ultra-high frequency waves is e.g. between 0.2 and9 GHz and that of the radio-frequency waves between 0.02 and 15 MHz.

Other features and advantages of the invention can be gathered from thefollowing purely illustrative and non-limitative description withreference to the attached drawing showing an embodiment of an apparatusmaking it possible to perform the inventive process.

BRIEF DESCRIPTION OF THE DRAWINGS

This drawing shows an enclosure 1 constituted by a cell 3 made from amaterial transparent to ultra-high frequency waves and a cavity 5 madefrom a conductive material, which may or may not be transparent toultra-high frequency waves. When cavity 5 is made from a conductivematerial, the part 6 of said cavity in the vicinity of cell 3 is madefrom an insulating material, e.g. quartz. The inner part 4 of the cellissues into the upper part of the cavity by means of a flared or widenedportion. The cell is e.g. made from quartz, whilst the cavity is e.g.made from aluminium or stainless steel.

Within the cavity is provided a support 7 able to receive one or moresubstrates to be etched. For reasons of simplicity, only one substrate 9is shown on the support and it is made from a single materialcorresponding to the layer to be etched. The widened portion 4 of thecell faces the substrate 9, the opening of said portion havingdimensions which are a function of those of the substrate 9.

The said apparatus comprises means for introducing a gaseous medium intothe enclosure. These means comprise several gas reservoirs 21a-21ncontaining different gases and connected by flow regulating valves 23and a duct 25a to the upper part of cell 3. There can also be differentducts for separately introducing the gases at different points of thecell, e.g. for introducing the fluorine-containing reactive gas at thesubstrate via duct 25b and for introducing the rare gas an O₂ in theupper part of said cell 3.

The apparatus also comprises pumping means 11, which can make itpossible to establish a vacuum in the enclosure and to circulate or notcirculate the gaseous medium introduced into the enclosure and extractthe gaseous medium resulting from the chemical reactions in theenclosure. Means 11 are connected to the lower part of cavity 5 by adiscontinuous annular slot 13 located in the lower wall thereof andcentered on support 7. These pumping means e.g. comprise a pump 15tightly connected by a toroidal duct 17 to the slot located in thecavity.

The apparatus shown in the drawing also comprises means for producingultra-high frequency waves in enclosure 1. These means comprise a firstultra-high frequency wave generator 27 connected by a waveguide 29 witha rectangular cross-section to the cell having a circular cross-section.Thus, the walls associated with the large sides of the waveguide have acircular opening permitting the passage of the cylindrical cell throughthe waveguide.

The cross-sectional dimensions of the waveguide are chosen as a functionof the frequencies of the ultra-high frequency waves used. At one of theends of the waveguide opposite to the ultra-high frequency wavegenerator is installed a piston 37 integral with a rod making itpossible to manually or automatically slide the piston over a certainlength. This length is preferably roughly the same as the wavelength ofthe waveguide propagation mode. This piston makes it possible toregulate the length of the waveguides in order to permit, as shownhereinbefore, a maximum, constant absorption of the power of theultra-high frequency waves transmitted to the gaseous medium by thewaveguide. An example of coupling a waveguide with a gas cell usable inthe apparatus according to the invention is e.g. described in FrenchPatents 2 534 040, 2 290 126 and 2 346 939, as well as U.S. Pat. No.4,049,940.

The apparatus also comprises means for producing radio-frequency wavesin the enclosure. These means comprise a radio-frequency wave generator31 connected on the one hand to a radio-frequency electrode 33 and onthe other to a counterelectrode connected to earth or ground. Electrode33 is constituted by part of the support 7. Thus, support 7 compriseselectrode 33 and a plate 40 made from an insulating material, such asquartz located on electrode 33. By raising substrate 9, said plate makesit possible to reduce the sheath phenomena. The radio-frequency wavesinteract with the gaseous medium of the enclosure in the vicinity of thesupport.

The counterelectrode is also constituted by the conductive walls of thecavity, but can also be constituted by a supplementary electrode 35(shown in dotted line form) located around electrode 33 and in thevicinity thereof, or by the walls of the cavity and the supplementaryelectrode. When a supplementary electrode is used, the latter is e.g.directly connected to the conductive walls of the cavity connected toearth and located at a limited distance from electrode 33.

In order to regulate the power of the radio-frequency waves, so as toobtain a maximum, constant absorption of the power of said waves in thegaseous medium, as stated hereinbefore, use is made of a tuning box 32connected between the radio-frequency wave generator 31 and theradio-frequency electrode 33.

Moreover, a cooling circuit 34 ensures a circulation of a cooling liquidwithin the electrode 33, so as to cool substrate 9.

The said apparatus can also comprise not shown means for regulating theheight of support 7 and therefore substrate 9 relative to the plasmacreated by the ultra-high frequency waves in the cell and relative tothe opening of the widened portion of said cell, in such a way that theinteraction of the ultra-high frequency waves with the gaseous mediumand the interaction of the radio-frequency waves with the gaseous mediumdo not disturb the propagation of these two plasma types.

There are also sealing means between the cell and the reactor, betweenthe reactor and the pumping means and between the radio-frequencyelectrode and the reactor.

As the reactor cavity is made from a conductive material, the latter isadvantageously covered by heating or plasma torch projection of acoating 42 not interacting with the plasma and over the entire innerwall thereof. The coating material is e.g. alumina (Al₂ O₃) with athickness between 300 and 500 μm. This treatment type for passivatingthe walls of the cavity with respect to the plasma created therein ise.g. described in French patent application 2 538 987.

Numerous modifications can be made to the apparatus shown in thedrawing. Thus, e.g. and as described in French patent application 2 534040, in certain cases, metal angles or wedges are advantageously addedin the vicinity of the cell to the waveguide.

This apparatus consequently makes it possible to etch a substrate withthe aid of a plasma produced by the interaction of an appropriategaseous medium with ultra-high frequency waves and radio-frequencywaves. This gaseous medium may or may not circulate in the enclosure viapumping means.

It can also be used for etching a substrate by a gaseous plasma producedsolely by ultra-high frequency waves and whilst eliminating thegenerator 31, etc.

In order to etch a silicon layer through a silicon dioxide mask, thesilicon layer covered by its mask is firstly placed on support 7. Thepressure is then established in the enclosure at a desired level bypumping means and the gases such as SF₆, argon, oxygen and optionallynitrogen at the desired flow rates are introduced into the cell via duct25, valves 23 and the different bottles or cylinders 21a to 21n. Thethus created gaseous medium is then made to interact with ultra-highfrequency waves produced by the ultra-high frequency wave generator 27and introduced into the cell by waveguide 29 and optionally withradio-frequency waves in the vicinity of the radio-frequency electrode33.

The following examples relating to the etching of polycrystallinesilicon are given in a non-limitative manner for illustrating theinvention.

EXAMPLE 1

In this example use is made of a gaseous medium formed by sulphurhexafluoride, oxygen and argon for etching by a plasma producedsimultaneously by radio-frequencies and ultra-high frequencies withrespective powers of 200 and 800W. Circulation takes place in theenclosure of sulphur hexafluoride at a rate of 15 cm³ standard/min,argon at 15 cm³ standard/min and oxygen at 5 cm³ standard/min under apressure of approximately 1 Pa and a temperature of 80° C. This leads toan etching rate Vg of 8 μm/min, a selectivity S of 10 and an anisotropyA of ≦0.05.

EXAMPLE 2

This example uses the same gaseous medium as in example 1 and it is madeto circulate in the enclosure with the same flow rates, but the plasmais formed solely by ultra-high frequency waves with a power of 800 W.This gives an etching rate Vg of 15 μm/min, a selectivity S of 64 and ananisotropy A of 0.52.

EXAMPLE 3

This example adopts the same operating procedure as in example 1, exceptthat the respective flow rates of SF₆, O₂ and Ar are those given in thefollowing table. The results given therein are obtained.

COMPARATIVE EXAMPLE 1

The same operating procedure as in example 3 is used for etchingsilicon, but using as the gaseous mixture a mixture of CF₄, O₂ and Arintroduced into the enclosure at a rate of 50 cm³ standard/min for CF₄,10 cm³ standard/min for O₂ and 30 cm³ standard/min for Ar. The resultsobtained are given in the following table.

These results make it clear that the replacement of SF₆ by CF₄ leads toinferior results. It is assumed that in this case the formation of astable non-volatile product on the surface of the silicon of the CF typeslows down the reaction of the silicon with the fluorine of thedischarge and leads to inferior results.

EXAMPLE 4

This example follows the operating procedure of example 1, but uses agaseous mixture comprising SF₆, Ar, O₂ and N₂. The respective flow ratesof the gases are 50 cm³ standard/min for SF₆, i.e. a partial pressure of1.62 Pa (12 mTorr), 30 cm³ s/min for Ar, i.e. a partial pressure of 0.81Pa (6 mTorr), 10 cm³ s/min for O₂, i.e. a partial pressure of 0.27 Pa (2mTorr), 10 cm³ s/min for N₂, i.e. a partial pressure of 0.27 Pa (2mTorr).

This gives a Si etching rate of 6 μm/min, a SiO₂ mask etching rate of0.03 μm/min, a selectivity S of 200 and an alpha angle of 70°.

EXAMPLE 5

The operating procedure of example 4 is adopted with flow rates of 50cm³ s/min (1.5 Pa) for SF₆, 32 cm³ s/min (1 Pa) for Ar, 8 cm³ s/min(0.35 Pa) for O₂ and 8 cm³ s/min (0.35 Pa) for N₂. Results equivalent tothose of example 4 are obtained.

                  TABLE                                                           ______________________________________                                                Example 3        Comparative Example 1                                ______________________________________                                        SF.sub.6 rate                                                                         50 cm.sup.3 s/min                                                     CF.sub.4 rate            50 cm.sup.3 s/min                                    O.sub.2 rate                                                                          10 cm.sup.3 s/min                                                                              10 cm.sup.3 s/min                                    Ar rate 30 cm.sup.3 s/min                                                                              30 cm.sup.3 s/min                                    Si etching                                                                            4.2 μm/min    0.35 μm/min                                       rate                                                                          SiO.sub.2 etch-                                                                       0.01 μm/min   0.14 μm/min                                       ing rate                                                                      Selec-  420              2.5                                                  tivity S                                                                      Homoge- Si ± 1.5%; SiO.sub.2 ± 1.5%                                                              Si ± 4%; SiO.sub.2 ± 11%                       neity                                                                         Anisot- 0.8              0.5                                                  ropy A                                                                        Surface <80 nm           <50 nm                                               roughness                                                                     ______________________________________                                    

A description will now be given of an apparatus for performing theprocess according to the invention in an illustrative, nonlimitativemanner with reference to the drawing.

The process according to the invention can be performed in all etchingmeans using a plasma produced by ultra-high frequency waves using thewaveguide-gas cell coupling mode (surfaguide) described hereinbefore,provided that use is made of a support able to support the said layer,which is at least in part conductive and optionally a radio-frequencywave generator connected on the one hand to the conductive part of thesupport and on the other to a counterelectrode connected to earth orconversely the layer on the electrode being connected to earth and thecounterelectrode to the radio-frequency generator. This counterelectrodeis constituted either by a supplementary electrode located in theenclosure, e.g. round the support, or by at least part of the enclosure,or by at least part of the enclosure and a supplementary electrode.

In certain cases, it is preferable to cool the walls of the enclosureconstituting the reactor. The apparatus also comprises appropriatecooling means, e.g. a circulation of water between the wall of cavity 5and a supplementary wall 45, in order to e.g. keep wall 5 at ambienttemperature.

We claim:
 1. In a process for etching a substrate with the aid of a gasplasma produced either by ultra-high frequency waves, or byradio-frequency waves and ultra-high frequency waves, the improvementcomprising that the gaseous medium used for forming the plasma comprisesat least one non-carbon-containing fluorinating gas, at least one raregas and at least one non-carbon-containing oxidizing gas.
 2. Processaccording to claim 1, characterized in that the gaseous medium alsocomprises at least one other gas chosen from among nitrogen andchlorine-containing gases.
 3. Process according to any one of the claims1 and 2, characterized in that the non-carbon-containing fluorinatinggas is sulphur hexafluoride SF₆.
 4. Process according to any one of theclaims 1 and 2, characterized in that the rare gas is argon or a mixtureof argon and helium.
 5. Process according to any one of the claims 1 and2, characterized in that the oxidizing gas is oxygen.
 6. Processaccording to claim 1, characterized in that the gaseous medium isconstituted by sulphur hexafluoride, argon and oxygen.
 7. Processaccording to claim 6, characterized in that the respective flow rates ofsulphur hexafluoride, argon and oxygen are 4 to 500 cm³ standard/min forSF₆, 4 to 500 cm ³ standard/min for Ar and 2 to 500 cm³ standard/min for0₂.
 8. Process according to claim 1, characterized in that the gaseousmedium is constituted by sulphur hexafluoride, argon, oxygen andnitrogen.
 9. Process according to claim 8, characterized in that therespective flow rates of sulphur hexafluoride, argon, oxygen andnitrogen are 4 to 500 cm³ standard/min for SF₆, 4 to 500 cm³standard/min for Ar, 1 to 300 cm³ standard/min for O₂ and 1 to 300 cm³standard/min for N₂.
 10. Process according to any one of the claims 1, 2and 6 to 9, characterized in that the substrate is of silicon. 11.Process according to any one of the claims, 2 and 6 to 9, characterizedin that etching is performed under a pressure of 0.1 to 300 Pa. 12.Process according to any one of the claims 1, 2 and 6 to 9,characterized in that etching is carried out by means of a gas plasmaproduced by ultra-high frequency waves and radio-frequency waves byconfining on the substrate by a cell made from a material transparent toultra-high frequency waves a gas plasma produced by transverse monomodecoupling between a waveguide traversed by ultra-high frequency waves andthe cell in which at least part of the gaseous etching medium circulatesperpendicular to the ultra-high frequency waves to be directed onto thesubstrate and by simultaneously applying to the substrate a d.c. or a.c.voltage having a frequency in the radio-frequency range.
 13. Processaccording to any one of the claims 1, 2 and 6 to 9, characterized inthat etching is carried out by means of a gas plasma produced byultra-high frequency waves by confining on the substrate using a cellmade from a material transparent to ultra-high frequency waves a gasplasma produced by transverse monomode coupling between a waveguidetraversed by ultra-high frequency waves and the cell in which at leastpart of the gaseous etching medium circulates perpendicular to theultra-high frequency waves to be directed onto the substrate.